Slices of rat hippocampus were exposed to 700 MHz continuous wave
radiofrequency (RF) fields (25.2-71.0 V m(-1), 5-15 min exposure) in a
stripline waveguide. At low field intensities, the predominant effect on the
electrically evoked field potential in CA1 was a potentiation of the
amplitude of the population spike by up to 20%, but higher intensity fields
could produce either increases or decreases of up to 120 and 80%,
respectively, in the amplitude of the population spike.

To eliminate the
possibility of RF-induced artefacts due to the metal stimulating electrode,
the effect of RF exposure on spontaneous epileptiform activity induced in
CA3 by 4-aminopyridine (50-100 &mgr;M) was investigated. Exposure to RF
fields (50.0 V m(-1)) reduced or abolished epileptiform bursting in 36% of
slices tested. The maximum field intensity used in these experiments, 71.0 V
m(-1), was calculated to produce a specific absorption rate (SAR) of between
0.0016 and 0.0044 W kg(-1) in the slices.

Measurements with a Luxtron
fibreoptic probe confirmed that there was no detectable temperature change
(+/-0.1 degrees C) during a 15 min exposure to this field intensity.
Furthermore, imposed temperature changes of up to 1 degrees C failed to
mimic the effects of RF exposure.

These results suggest that low-intensity
RF fields can modulate the excitability of hippocampal tissue in vitro in
the absence of gross thermal effects. The changes in excitability may be
consistent with reported behavioural effects of RF fields.